Inhibition of the gastric protease, pepsin, by a variety of peptide fragments will be studied. The critical structural amino acids will be identified by a process of selective synthesis of replacement analogs. The binding of these peptides to pepsin will be characterized by inhibition of pepsin catalysis, circular dichroism studies, fluorescence changes of dansylated inhibitors, kinetics and thermodynamics of inhibition, and affinity labelling procedures. The results of these studies will be integrated into a coherent mechanism for inhibition of pepsin. This information will be used to plan the synthesis of more effective inhibitors. We will explore the use of a new peptide substrate with the advantages of solubility, a spectroscopic assay method, and unique enzymatic hydrolysis products.